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Partnership showcases Perpetual Pavement project

At $105 million, the highway reconstruction project on Highway 401 near Woodstock, ON. is among the largest single projects in MTO history.

At $105 million, the highway reconstruction project on Highway 401 near Woodstock, ON. is among the largest single projects in MTO history.

The $105 million, six lane project on Highway 401 near Woodstock, ON. is among the largest single highway  reconstruction projects in MTO history.

The six lane, 15.3a km long projects involves over 400,000 tonnes of hot mix asphalt, and will some three years to complete from its summer 2008 start. Aecon Construction & Materials Ltd. is the general contractor and Capital Paving Inc. is a principal subcontractor as hot mix asphalt supplier and paving contractor. The project’s partners include the Ontario Ministry of Transportation (MTO), the Centre for Pavement and Transportation Technology and the Ontario Hot Mix Producers Association.

Conduit is used on the Highway 401 Perpetual Pavement project to pull data transmission cables through to the data logger.
Capital Paving’s crews will place over 400,000 tonnes of hot mix asphalt on the Highway 401 perpetual pavement project near Woodstock ON.
A perpetual pavement test section has been constructed at Capital Paving’s asphalt plant, complete with all the instrumentation used on the Highway 401 project.  Data from this controlled environment will be monitored and compared to Highway 401 site.
MASc candidate Mohab El Hakim explains the application of a thermistor string, used to collect the temperature of all pavement layers.

Pavement design
Becca Lane of the MTO explained that two sections, each some 2km long, at either end of the project will be constructed utilising perpetual pavement designs, while the central 11km section will utilise a conventional design.

Perpetual Pavement is a thick flexible pavement consisting of a renewable surface layer, a strong rut-resistant intermediate layer and a flexible fatigue-resistant bottom layer. Unlike thin pavements that readily crack and rut under repeated loading of heavy traffic, the potential for fatigue cracking in thick asphalt pavements is reduced and pavement distress occurs only in the top layer. When the surface distress reaches a critical level, the top layer is removed and replaced. The ride remains good for most of a perpetual pavement’s service life, resulting in a high level of serviceability and higher road user satisfaction.

On this particular project, the perpetual pavement cross-section on mainline reconstruction or rehabilitation typically includes 420 mm of hot mix asphalt, 200mm of Granular A and 550 mm of Granular B base material. The asphalt lifts include the 40mm Superpave 12.5 FC 2 surface layer; 50mm SP19.0, 60mm SP 19.0, 70mm SP 19.0, and 100mm SP 25.0, together making up the intermediate layer. For the flexible bottom layer, one of the perpetual pavement sections will utilise a 100mm rich bottom mix (RBM) while the other will utilise 100mm of conventional SP 25 mixture. Rich bottom mix is a flexible, fatigue resistant hot mix asphalt designed with an increased asphalt cement content and reduced air voids to improve flexibility and minimize bottom-up cracking. Granular base materials beneath the asphalt on both perpetual pavement sections include 200mm of Granular A and 550mm of Granular B Type III.

Pavement instrumentation
Susan Tighe of CPATT explained that the MTO and Centre for Pavement and Transportation Technology (CPATT) at the University of Waterloo have partnered in a research project to assess pavement performance on the project.

Three pavement sections including the perpetual pavement reconstruction, perpetual pavement rehabilitation, and convention pavement sections will be equipped with instrumentation to improver our understanding of how perpetual pavements perform and deteriorate, taking environmental and traffic impacts into account. Pavement performance will be monitored through the collection of strain, vertical pressure, temperature and moisture data. 

Earth Pressure Cells (EPC) were installed under the wheel path in order to measure the vertical pressure on the top of subgrade. In addition, moisture probes (MP) were installed to measure the moisture content in the subgrade layer, recognising that moisture content in the subgrade layer plays a significant role in pavement deterioration and performance. Asphalt Strain Gauges (ASG) are installed to measure strain in the critical zones where cracks initiation is expected, whereas Thermister strings (TS) are installed to monitor the temperature of different pavement layers. Weigh-In-Motion (WIM) sensors will be installed to capture vehicle axle loads at a later date.

During construction, earth pressure cells and moisture probes are placed in the subgrade & granular materials while the asphalt strain gauges are placed in the asphalt. The thermister strings are placed right at the end after final surface course is placed, as are the weigh-in-motion sensors. Conduit is used to pull the cables through to the data logger. Once the pavement is in service, it is expected that the asphalt strain gauges will capture crack initiation at the bottom of the asphalt concrete layer under the wheel path.

Asphalt supply and paving
Mark Latyn and Rob Munro of Capital Paving Inc offered some insight into the contract’s challenges from the asphalt supplier and paving subcontractor’s perspective. 

Overall, Latyn noted that the biggest challenge is in maintaining product quality while dealing with the sheer volume of asphalt involved. 

On the materials side, for instance, RAP from the Highway 401 job must be stockpiled separately from other sources due to its 100% crushed requirement, thereby requiring additional RAP feeder bins at the asphalt plant. Still on materials, Lafarge’s Dundas is supplying aggregates for all base mixes and, as big league player, understands the risks involved on a project of this scale. Dundas’ dolomitic limestone is resistant to stripping, although the stone does have a high affinity for asphalt and is somewhat prone to breakdown in the dryer drum.

To ensure material consistency, large stockpiles of all aggregates are being maintained in Capital’s yard, with same day delivery avoided wherever possible. Two loader operators are required for safe operation, with

deliveries scheduled during slower periods and QC samples taken daily during delivery to ensure consistent supply quality. 

On the asphalt production side, logistical issues arise when producing large quantities of the project’s Superpave mixes along with day-to-day Marshall mixes. On previous contracts, most MTO paving work of the work has been done at night, negating the need to switch mixes. It is a different story on this contract, paving day and night and running at over 2,000 tonnes per shift, all while supplying the company’s other two crews as well as outside sales. In these circumstances, plant mix timings are critical to prevent excessive waste while switching mixes. Close communication by the asphalt production crew is essential to keep loader operators informed about aggregates and mixes, and everyone must be aware of what materials are in the cold feed bins at all times. Latyn adds that sometimes even the twelve bins at Capital’s plant are not enough.

Controlling dust, that is material passing the 0.075 mm sieve, is important as coarse mixes are sensitive to even small changes in asphalt cement (AC) content and dust content. Fortunately there are several ways to control dust, including the use of quality and washed aggregates, reducing the RAP content that typically accounts for 8-10% of dust, reducing the quantity of unwashed screenings, and either removing or metering back dust addition. 

On the quality control (QC) side, job challenges include the 24 hour staffing of Capital’s laboratory. This has necessitated considerable staff training and Latyn places attitude high in the list of requirement for new technicians. Capital has hired a full time full time QC “run-around” person and is achieving the two day turnaround times while continuing to test day-to-day Marshall mixes. As it takes six to eight hours to complete Superpave material testing from a plant that is producing 350-400 tonnes/hr, there is no break in the action and little time to review or make mix changes. This is just one of the reasons why Latyn describes a QC Field Technician as “invaluable” and the first line of defence on projects of this magnitude.  

Turning to construction, Munro noted that co-ordination with general contractor Aecon has been excellent. Site problems on a project of this scale rarely involve asphalt quality, but the staging of multiple asphalt lifts plays a critical role in job progress. If, for instance, the paving crew aims for a typical lap joint of 200mm, then 800mm of room is required to step these joints back. As a result, joints may fall within the wheel path which in turn can potentially cause smoothness problems and premature joint deterioration. 

Maintaining a steady supply of asphalt to paving crews can also be a major concern, with variables such as the weather, accidents or “normal” traffic congestion all capable of upsetting the best laid plans and the timely delivery of material to site. Site design requirements can also impact work staging. The project’s bridge decks, for example, are designed to have two lifts of SP 12.5 FC2 surface asphalt mix and as the decks often have to be completed one side at a time, their paving often presents a further challenge to production planners.

Munro concluded with some insight on the all important human factor. “Due to the project’s duration, Capital changes paving crews whenever possible to prevent crew burn out related to night shifts. If we rotate the crews around, then all of our available crews gain the knowledge and experience from a contract such as this. All these factors promote a better quality product on this project and others.”